Japanese Patent Early Publication No. 6-111987 discloses a prior art electronic ballast for a high intensity discharge lamp with a dimming capability. The ballast includes a power converter which converts an input DC voltage into a lamp power for driving the lamp, a dimmer providing a dimmer command of a varying dimming ratio, and a controller which controls the power converter to reduce the lamp power as the dimming ratio decreases. A voltage monitor is included to monitor a lamp voltage across the lamp as indicative of a lamp characteristic during the dimming operation. While the lamp power is reduced to a certain level, i.e., the dimming ratio decreases from 100% down to a relatively high ratio, the controller makes a constant wattage control of varying a lamp current in accordance with the lamp voltage being monitored so as to supply a roughly constant lamp power to the lamp. As the dimming ratio decreases further, which is acknowledged by the lamp voltage exceeding a critical level, the controller switches to a constant current control of supplying a constant current only determined by the dimming ratio and not by the monitored lamp voltage in order to dim the lamp to a deeper extent without causing a lamp extinction.
The above constant wattage control during the lamp dimming is realized based upon an assumption that the lamp voltage remains substantially constant while the lamp power is reduced from a rated power to the certain level at which the lamp voltage turns to increase beyond the critical level. That is, the controller is designed to have a voltage-wattage output characteristic which gives generally quadratic function curves respectively for varying dimming ratios (a), (b), and (c), as shown in FIG. 16. These curves, which are generated in accordance with voltage-current relations determined respectively for different dimming ratios, as shown in FIG. 17, have individual apexes aligned along a fixed lamp voltage to provide an operable voltage range around the apex for each curve. The operable voltage range defines a voltage range within which a lamp characteristic curve Q1 crosses the output curves (a), (b), and (c) for operating the lamp while keeping the resulting lamp power at roughly the same level even in the presence of a possible variation in the lamp characteristic Q1. That is, when the lamp characteristic Q1 curve intersects the output curves (a), (b), (c) within the operable voltage range, only a small deviation in the lamp power is seen to assure the constant wattage control for dimming the lamp to a reduced lamp power as intended, irrespective of a possible variation in the lamp characteristic that may be caused by a lamp manufacturing process.
Although the prior art ballast teaches the constant current control for successfully dimming the lamp to a deeper extent, the above constant wattage control is not satisfactory for dimming the lamp consistently due to the fact that the lamp characteristic during the lamp dimming does actually follow a curve Q rather than Q1 as assumed in the prior art ballast. That is, the lamp voltage will decreases as the lamp power reduces from its rated power to a certain level, for example, 40% of the rated lamp power. With this result, an even slight variation in the lamp characteristic (shown for example by a curve Q2 in FIG. 16) brings about a considerable variation in the lamp power. Such considerable lamp power variation has to be avoided particularly when two or more discharge lamps are simultaneously dimmed since the lamp characteristic variation results in an appreciable lamp power difference although they are intended to be dimmed to the same level.